Cellulose ethers have widespread application in thickening agents (e.g. in food additives), bonding agents (e.g. in lacquers and other paints), adhesives, printing pastes, suspension stabilizing agents, thermoplastic materials, protective colloids, emulsion stabilizers, finishing compositions (e.g. in textiles), coating compositions (e.g. in paper and paper products), plastic sheets (e.g. in packaging or textiles), and film-forming agents. See, for example, Cellular Materials to Composites, Volume 3, Chapter “Cellulose Ethers”, pages 226-269, Encyclopedia of Polymer Science and Engineering, 2nd Ed., 1985, John Wiley & Sons, New York. Many of these applications benefit from the relatively high viscosity of cellulose ethers (e.g. petroleum production fluids). Prior to the present invention, the solution viscosity of cellulose ethers have primarily been increased by increasing the degree of polymerization (DP) or viscosity of the cellulose pulp used to make the cellulose ethers, or by protecting against DP or viscosity degradation during pulp grinding and other processes performed during cellulose ether manufacturing.
For example, if a specific solution viscosity is desired for the cellulose ethers, the appropriate cellulose pulp viscosity is determined and a cellulose pulp having this viscosity is selected. This raw material selection strategy is also employed by ether producers to increase production throughput by using higher density cellulose floc.
Cellulose ethers are typically produced by alkalating cellulose with an alkalating agent, such as sodium hydroxide, to form an alkali cellulose and then etherifying the alkali cellulose. See, for example, U.S. Pat. Nos. 2,067,946; 2,636,879; 4,063,018; 4,250,305; 4,339,573; and 4,547,570. The cellulose pulp may be shortened or granulated into a cellulose floc before being alkalated. See, for example, U.S. Pat. Nos. 2,067,946; 2,636,879; and 4,339,573.
Edelman et al., U.S. Pat. No. 4,941,943, disclose a pretreatment process for preparing sodium carboxymethyl cellulose. The pretreatment process includes slushing cellulose to a consistency of about 5-15% to form a fibre suspension, concentrating the fibre suspension to a consistency of about 25-35% to form a pulp, and homogenizing the pulp. After the fibre suspension has been concentrated, the pulp is mercerized to form activated cellulose (or alkali cellulose). The mercerization step may occur before, after, or concurrently with the homogenizing step. After the pretreatment process, the activated cellulose is etherified to form the sodium carboxymethyl cellulose.
Orii et al., U.S. Pat. No. 4,292,426, disclose a process for preparing hydroxylpropyl cellulose. The process requires swelling cellulose, driving out excess caustic soda solution from the swollen cellulose to form a dehydrated cellulose, and obtaining an alkali cellulose by washing with an aqueous caustic solution and drying the dehydrated cellulose. The alkali cellulose is washed in order to reduce the alkali content of the dehydrated cellulose. The resulting alkali cellulose has a sodium hydroxide to cellulose ratio of 0.05 to 0.16 and a water to cellulose ratio of 0.2 to 0.5. The process further comprises reacting the alkali cellulose with propylene oxide until the molar substitution is between 1 and 2, adding 0.2 to 0.7 parts of water for every one part of cellulose in the raw material alkali cellulose, and continuing the etherification reaction until the molar substitution is higher than 2.5.
Bredereck et al., U.S. Pat. No. 4,491,661, disclose a process for preparing water-soluble cellulose ethers comprising activating cellulose with ammonia, alkalizing the activated cellulose with an alkalizing agent in the presence of ammonia, removing the ammonia from the alkali cellulose in the presence of the alkalizing agent, and etherifying the alkali cellulose with an etherifying agent in the presence of an organic solvent. When cellulose is activated with liquid ammonia, crystalline cellulose III is formed. As well known in the art, crystalline cellulose III has a significantly different structure and properties than other crystalline cellulose structures, such as crystalline cellulose II. See, for example, Rydholm, supra; and Ott, Spurlin and Grafflin, supra.
Laskowksi et al., DDR Patent No. 146,462, disclose a process for preparing low substituted, water soluble carboxymethyl cellulose having a degree of substitution less than 0.5. The process includes activating cellulose with liquid ammonia, steaming off the ammonia, alkalizing the activating cellulose with sodium hydroxide, and carboxymethylizing the alkalized cellulose.
Dautzenberg et al., DDR Patent No. 148,342, disclose a process for preparing carboxymethyl cellulose with increased solution viscosity. The process includes activating cellulose with liquid ammonia, alkalizing the activated cellulose, and carboxymethylizing the alkalized cellulose.
There is a need for a method of preparing cellulose ethers which increases throughput and in which the solution viscosity of the cellulose ethers may be controlled by process conditions, not just the viscosity of the starting raw materials. There is also a need for a method of preparing cellulose floc which increases the bulk density per number average floc length.